1. Field of the Invention
The present invention relates to coupled resonator filters. More particularly, the present invention relates to coupled resonator filters formed by micromaching techniques and a method for making the same.
2. Related Art
Microwave filters are devices that pass some microwave signals and reject others, based on the frequency of the signal. Conventionally, microwave filters are made using resonate structures in different arrangements. Depending on the arrangement of the resonant structures, filters can be made which are low-pass (i.e., pass lower frequencies and reject higher frequencies), high pass (i.e., pass higher frequencies and reject lower frequencies), band pass (i.e., pass a limited range of frequencies), and band reject (i.e., reject a limit range of frequencies).
Filters can also be combined to form other devices, such as diplexers. Diplexers take an input signal, and send one band of frequencies to one output, and another band of frequencies to another output. Other configurations of this type are possible, although less commonly used.
There are many ways of evaluating the electrical performance of a bandpass filter. Different characteristics will be important, depending on the application that the filter is being used in. For example, insertion loss describes the amount of energy lost in a signal that is supposed to be passed, rejection describes the amount of energy that is supposed to be blocked at a given frequency, but which is passed through, and passband flatness describes the variation of insertion loss over the range of frequencies which are passed. These various parameters are mutually dependent on each other. For example, better rejection can be designed at the expense of either passband flatness or insertion loss.
There is, however, a fundamental figure of merit for a resonator filter which defines how good a filter can be made from a series of similar resonators. This figure of merit is known as the quality factor, or Q, of the resonator. Roughly, it is the inverse of the fraction of energy lost for each oscillation of the resonator. For a technology with a higher resonator Q, a filter with better characteristics can be made than a filter with lower Q. For example, a higher resonator Q filter could be made with better insertion loss than a filter with lower Q resonators, with all other characteristics being the same.
Conventionally, higher Q filter technology utilizes machined cavities in metal as the resonator structure. However, because conventional machining is not precise enough to exactly give the desired filter response, tuning structures such as screws are needed to “tune up” a filter to give it an optimized response. A disadvantage of this type of structure is that it requires manual adjustment of the filter by a technician to give the optimum response.
The most common type of micromachined filter utilizes thin lines of suspended metal. The metal can either be strips attached at either end and hanging suspended, or an arbitrary shaped pattern on a very thin dielectric. By removing the surrounding dielectric, the resonator Q is improved.
However, this type of filter suffers from the thinness of the resonator line (the deposited metal is usually no more than a few microns thick) and the edge coupling of the resonators. This concentrates the currents along the edge of the resonator, which increases the ohmic losses, and reduces the resonator Q. This design also can be susceptible to small mechanical vibrations known as microphonics, which can modulate a signal passed through the filter.
Therefore, what is needed is a filter design having the advantages of higher resonator Q, which translates into lower insertion loss and better rejection, while also exhibiting an immunity to microphonics and a reduced manufacturing complexity.